If you're going to build a computer from scratch for a specific purpose, you'll want to keep that purpose in mind when choosing your components; don't just go to the store or an online shop and start buying components. First, decide what you want to use the computer for. The reason you have to consider this is that you may be able to save money by only specifying expensive, premium components where needed.

Source : http://en.wikibooks.org/wiki/How_To_Build_A_Computer